DISSOCIATION OF THE ELECTRICAL AND CONTRACTILE PROPERTIES IN SINGLE HUMAN MOTOR UNITS DURING FATIGUE

The surface EMG area often exhibits progressive enlargement during a s ubmaximal fatiguing contraction, but the underlying reasons still rema in uncertain. Fatigue-induced changes in the surface-detected motor un it action potentials (S-MUAPs) of 10 human thenar motor units (MUs) wi th widely differing physiological properties were examined. After 2 mi n of repetitive 40-Hz stimulation, the size of the S-MUAPs of all MUs increased, the magnitude of which was negatively correlated with their tetanic tension changes. These findings suggest that during muscle fa tigue, in addition to reflecting recruitment of new MUs and increases in firing rates of the active MUs, the surface EMG may also be markedl y influenced by changes in the S-MUAPs, especially in fast fatigable m uscles, (C) 1998 John Wiley & Sons, Inc.